1. Field of the Invention
This invention relates to a loudspeaker apparatus. More particularly, it relates to a loudspeaker apparatus employing a motional feedback technique.
2. Background of the Invention
In general, a loudspeaker apparatus employing a motional feedback (MFB), referred to hereinafter as MFB speaker, detects vibrations of a vibrating system and feeds the vibrations back to an amplification circuit to form a feedback loop for driving the vibrating system. It is thereby possible with the MFB speaker to reproduce audio signals with high sound quality.
With the MFB speaker, driving signals are supplied to a voice coil arranged in a gap of a first magnetic circuit for vibrating a diaphragm connected to the voice coil in accordance with driving signals.
Besides, with the MFB speaker, vibrations of the voice coil are transmitted to a detection coil arranged in a second magnetic circuit so that vibrations of the diaphragm and the voice coil are detected by the detection coil.
Thus the MFB speaker provides a feedback loop of feeding back an output signal of the detection coil to an amplification circuit.
It may now be contemplated that, with the MFB speaker, the overall size of the speaker may be .reduced if the voice coil and the detection coil are coaxially arranged by the application of a coaxial two-way speaker system as disclosed for example in JP Utility Model Kokai Publication No.57-43437.
However, if the voice coil and the detection coil are coaxially arranged by simply applying the coaxial two-way speaker system disclosed in the above-cited Publication, it may occur that the detection coil is vibrated in a wobbling manner in case of vibrations of the voice coil displaced linearly in the axial direction.
Thus it becomes difficult to detect the vibrations of the vibrating system by the detection coil so that high-quality audio signals cannot be reproduced.
It may now be contemplated to have the voice coil and the detection coil held coaxially with the use of a sturdy holding member for reducing wobbling of the detection coil.
However, if the holding member is of a sturdy structure, the vibrating system is increased in mass weight as a result of which the response characteristics of the loudspeaker apparatus is deteriorated.
FIG. 1 shows basic detected MFB signals. Since the detection signal of the MFB speaker is the measured value of the vibrating speed of the diaphragm, an output level is decreased from a maximum value in the vicinity of f.sub.0. As the frequency characteristics of the detection signal, measurement is made of the resonant peak of a connecting portion between the voice coil and the detection coil.
Consequently, if the resonant level approaches an output level set for MFB, a detection signal deviated from a pre-set phase is applied to an output side, so that the problem of oscillation is raised.
Attempts have been made to eliminate such oscillation by a vibration damper, such as rubber damper, to a coupler for suppressing resonant peaks. However, the vibrating system is increased in mass weight thereby affecting speaker characteristics and increasing the number of process steps.